A computer system, and other processor based systems, are known to include a central processing unit, video graphics circuitry, system memory, and other peripheral circuits. In such systems, the central processing unit functions as a host processor while the video graphics circuitry (e.g., a graphics co-processor) functions as a loosely coupled co-processor. By way of example, the video graphics circuitry may be an integrated circuit on a single semiconductor die, such as an application specific integrated circuit (ASIC). Additionally, the video graphics circuitry may include memory, such as, but not limited to, dynamic ram (DRAM). This memory may reside on the same semiconductor die (ASIC) as the video graphics circuitry or it may be separate and connected through board level or package level traces.
In conventional computer systems, a clock generator circuit, such as a phase lock loop (PLL) circuit generates a clock signal for the video graphics circuitry. As is known in the art, each device, such as the video graphics circuitry and the memory, may also include individual phase lock loop circuits for generating a respective clock signal. In these conventional computer systems, the frequency of the clock signal for the computer system is set at a maximum operating clock frequency to ensure suitable computer system performance at the maximum rated operating temperature. Manufacturers establish a nominal clock frequency of the computer system, such as the video graphics circuitry, via a qualification process. According to the qualification process, the maximum operating clock frequency is established based on the video graphics circuitry operating at a maximum junction temperature. As a result, the nominal operating frequency of the clock signal is established based on a worst case operating condition of a maximum junction temperature. The maximum operating junction temperature may be determined by detecting the junction temperature or, alternatively, by detecting the case or package temperature and calculating the junction temperature based on heat transfer equations as is known in the art. Accordingly, junction temperature, as used herein, may also include case temperature.
The nominal operating frequency is established at the maximum junction temperature based on the signal propagation and timing requirements of the individual circuits within the video graphics circuitry, and the ability of the video graphics circuitry chip to dissipate heat, such as through a heat sink. The power dissipation characteristics of the video graphics circuitry and other integrated circuits are typically related to the operating frequency of the corresponding clock signal. As the operating frequency of the clock signal increases, the video graphics circuitry tends to generate more heat. As a result, the junction temperature generally increases as the clock frequency increases. Consequently, as the junction temperature increases, in order to ensure proper operation of the video graphics circuitry, the highest possible clock frequency permissible while decreases.
According to one method, the video graphics circuitry is shut down if the junction temperature on the video graphics circuitry exceeds the maximum junction temperature permitted for proper operation of the video graphics circuitry. However, such a method is limited to a safety mechanism for preventing the junction temperature from exceeding the maximum established junction temperature that may otherwise cause permanent damage to the video graphics circuitry. For example, if a cooling fan coupled to the video graphics circuitry fails or if a heat sink to the video graphics circuitry falls off, the video graphics circuitry may be shut down to prevent permanent damage to the video graphics circuitry.
According to another method, the clock frequency of the video graphics circuit, and other integrated circuits, has a fixed frequency clock that can be manually changed by the user through a graphical user interface. Nevertheless, the clock frequency is typically factory set upon initialization and often does not change. However, since the clock generation circuit is typically a phase lock loop, when the user manually changes the clock speed, such as during BIOS setup, a phase lock loop in a clock generator circuit is reprogrammed. Reprogramming of the phase lock loop may result in abruptly changing the clock speed. Such an abrupt change in clock speed may also result in the video graphics circuitry causing an undesired flicker in a resulting video graphics display. Accordingly, such a change in clock speed may only occur during manual setup, such as during BIOS setup, and not dynamically during graphics processing.